The invention is based on a method and an apparatus for determining the injected fuel quantity in internal combustion engines, as generally defined hereinafter. Measures for ascertaining the injection quantity in Diesel engines, that is, internal combustion engines having self-ignition, are known in many forms. One of the known possibilities encompasses a diagnostic device (German Offenlegungsschrift No. 21 43 676), which has a piezoelectric pressure measuring transducer in the pressure line from a fuel pump to the injection nozzle, the electrical output signals of the transducer then being used for indicating the instant of injection, injection pressure or injection frequency. In a similar manner, in a further known apparatus (U.S. Pat. No. 3,511,088), a piezoelectric converter is used, in order to attain, among other things, data as to the quantity of fuel delivered to the combustion chambers of an internal combustion engine. In all the known apparatus, the criterion is pressure and the variation of this pressure in the fuel line leading to the injection valves. This may lead to disadvantages because it has been found that the evaluation of the individual pressure signals cannot always produce satisfactory results and, further, because errors in measurement, caused for example by the effects of resonance in the pressure lines or by other stray interference, are unavoidable.
On the other hand, particularly precise ascertainment of the injection quantity in internal combustion engines, and in particular in Diesel engines, is increasingly desirable, first because such values are increasingly used for regulating purposes in regulating the onset of injection of an injection nozzle or the onset of supply by the fuel pump, but also in order to be able to furnish a load signal for exhaust gas recirculation systems. Such load signals are indispensable for satisfactory control of the recirculated quantity of exhaust gas in Diesel engines; and finally, signals proportional to the injected fuel quantity are also required for fuel consumption gauges.